Free radical induced radiation curable coatings and films are normally formulated with acrylate monomer diluents and acrylate functional oligomers. The formulations usually contain minor amounts of additive ingredients such as surfactants, slip agents, defoamers, thickeners, and/or thixothopes. Representative oligomers are the acrylate functional end capped urethane polyesters, polyols and acrylate functional end capped bisphenol A epoxy and novalic epoxy resins.
Since acrylates are not conducive to cationically induced radiation curing, they require free radical systems which are oxygen inhibited unless reacted in an inert atmosphere, generally under a blanket of nitrogen. Although formulation with a photoinitiator which undergoes bimolecular reaction with a hydrogen donor minimizes the inhibitory effect of air, this benefit is realized at the expense of a greatly reduced cure rate. Also, it is found that polymerization or curing in free radical systems ceases almost immediately upon removal from the source of radiation; thus, the cured product often contains some unpolymerized components. Accordingly, it is an aim of research to develop an oligomer having the beneficial properties of acrylates but which is amenable to radiation curing at a rapid rate by cationically induced polymerization which is not oxygen inhibited and which permits continued polymerization after removal from the source of radiation exposure.
It has also been found that acrylate formulations when stored under normal condition require the addition of a free radical scavanger such as substituted hydroquinones and phenothiazine to achieve long term stability. However, after the stored coating is applied on a substrate, the inhibitory effect of the stabilizer significantly reduces the cure rate.
Finally, it is noted that the unsubstituted acrylates are sensitizers and skin irritants as well as being carcinogenic, so that specialized safety precautions must be taken to protect operators from exposure. Although alkoxylation has lessened irritancy of the acrylates, their carcinogenic properties are not reduced.
Accordingly it is an object of the present invention to overcome the above described deficiencies by employing an economical and commercially acceptable compound or composition and curing process.
Another object of this invention is to utilize a multifunctional cross-linking agent, which is itself a polymerizable viscous liquid and which assists rapid radiation curing when formulated with allyl, epoxide or acrylate monomers and oligomers.
Another object is to provide a non-toxic cross linkable compound which is suitably cured as a film or as a coating on a substrate and which possesses good adhesion, high abrasion resistance and resistance to chemical attack in both acid or basic media.
Still another object is to provide a more economical process for cross-linking monomeric or polymeric acrylates or epoxides within a few seconds which can be effected in the presence of air.
Another object is to provide a compound which is curable at a rapid rate by cationically induced radiation.
Yet another object is to provide a substrate coated with a rigid scratch resistant and chemical resistant coating.
These and other objects will become apparent from the following description and disclosure.